This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
The invention is described with some references to LTE, but it should be noted that it conceivably can be applied to other systems as well, including e.g. WCDMA. It should also be noted that the invention is equally applicable both in the downlink as well as the uplink of a wireless system.
Signal and channel quality estimation is a fundamental part of a modern wireless system. The noise and interference is used not only in the demodulator, but is also an important quantity when estimating, for example, the channel quality indicator (CQI). The following is noted with LTE in mind, but the principles are valid for most wireless systems.
The interference can be estimated from the common reference symbols (RS) that are present in the time—frequency grid of an OFDM based system (e.g., LTE). An element in the time—frequency grid is referred to as a resource element (RE). The received signal in a RE can be written as r=Hs+n, where H is the channel response and s represents the transmitted symbols. The term, n, is the (unknown) noise and interference. The noise and interference show different characteristics in resource elements (RE) that hold RS, data, and control signaling; we therefore denote the received noise/interference for the different signaling types by I_RS, I_d, and I_control, respectively.
It is noted that the interference term in a RE containing RS, I_RS, at a specific user equipment (UE) can be estimated by the UE since s are known symbols and H is given by the channel estimator. It is further noted that the interference on REs with data (that is scheduled for the UE in question), I_d, also can be measured as soon as the (data) symbols, s, are detected (at this moment they can be regarded as known symbols). Similarly, to estimate I_control, the control symbols must first be detected.